1. Field of the Invention
This invention relates to a display panel comprising organic electroluminescence elements (designated as organic EL elements hereinafter).
2. Discussion of Related Art
An organic EL element is well-known which comprises a substrate consisting of a glass plate or a transparent organic film and a fluorescent layer formed on the substrate, wherein a current passing through the fluorescent layer causes the fluorescent layer to emit a light.
FIG. 1 shows a schematic diagram showing one embodiment of the above organic EL element. Referring to FIG. 1, an organic EL element includes a glass substrate 1, a transparent electrode 2 formed on an upper surface of the glass substrate 1 and a light emitting layer 3 formed on an upper surface of the transparent electrode 2. The organic EL element further includes a metal electrode formed on an upper surface of the light emitting layer 3.
FIG. 2 shows an electric equivalent circuit representative of the organic EL element. As shown in FIG. 2, the organic EL element is generally considered to be a capacitive light emitting device which equivalently includes a circuit resistance component R, a capacitive component C, and a light emitting component D.
Accordingly, when a driving voltage for the light emission is applied to the organic EL element, a flow of electric charge corresponding to the electric capacitance of the element is passed into the electrode as a displacement current to store therein. It is considered that if a potential induced in the electrode then exceeds a predetermined voltage (barrier voltage), a current flow begins to flow into the organic layer, so that emission from the organic layer starts proportionally to the quantities of the current flow.
FIG. 3 shows one embodiment of a display device including a plurality of organic EL elements. As shown in FIG. 3, such a device comprises a cathode-line scanning circuit 51, an anode-line driver circuit 52, and a display panel (not shown). The cathode-line scanning circuit 51 is connected to the display panel through a plurality of connecting terminals b1-bn which consist of connecting sections. Similarly, the anode-line driver circuit 52 is connected to the display panel through a plurality of connecting terminals a1-am which consist of connecting sections.
A method for driving the display panel with the circuit shown in FIG. 3 is called a simple matrix driving method, which comprises the steps of;
placing anode-lines A1-Am and cathode-lines B1-Bn in a matrix (lattice) arrangement,
connecting a cross point of the anode-line and the cathode-line in the matrix arrangement to the corresponding one of the light emitting elements E1,1-Em,n,
selecting either of the anode and cathode lines to scan each of the selected ones in order at regular time intervals, and
driving the other lines with current sources 521-52m as power supplies in synchronization with the above scanning, whereby any light emitting elements on the corresponding cross points can emit a light.
There are two types of methods for driving the organic EL elements, i.e. cathode-line scanning and anode-line driving technique, and anode-line scanning and cathode-line driving technique. FIG. 3 shows the constitution for the cathode-line scanning and anode-line driving technique. The circuit of FIG. 3 comprises the cathode-line scanning circuit 51 connected to the cathode-lines B1-Bn, and the anode-line driving circuit 52 with the power sources 521-52m connected to the anode-lines A1-Am. The cathode-line scanning circuit 51 causes switches 531-53n to switch to their ground terminal in order at regular time intervals to scan the cathode-lines B1-Bn, so that ground potential (OV) is applied to the cathode-lines B1-Bn in order. The anode-line driving circuit 52 controls the switching of the switch 541-54m in synchronization with the switch scanning of the cathode-line scanning circuit 51 to connect the power sources 521-52m to the anode-lines A1-Am, so that a driving current flow is supplied to a desired light emitting element on a cross point.
The following description is made for explaining the light emission of the light emitting elements E2,1-E3,1. As shown in FIG. 3, when the switch 53, in the cathode-line scanning circuit 51 is switched to the ground terminal and the ground potential is applied to the first cathode-line B1, the switches 542 and 543 in the anode-line driving circuit 52 is switched to the terminal connected to the power source, and the anode-lines A2 and A3 is then connected to the power sources 522 and 523, respectively, so that the light emitting elements E2,1-E3,1 can emit light. The rapid repetition of the scanning and driving described above causes the light emission of any given light emitting element, and the light emitting elements are controlled in the manner that they seems to simultaneously emit light.
In order to prevent erroneous light emissions of the remaining light emitting elements, an inverse bias voltage Vcc which has the same magnitude as the voltage of the power source and the opposite polarity to that of the power source is applied to the cathode-lines B2-Bn except for the scanned cathode-line B1. Although the circuit of FIG. 3 uses the current source 521-52m as driving sources, it should be understood that voltage sources can be used as the driving sources.
The organic EL elements described above use a light emitting display function to constitute a display. Information signals are received into the display, by capturing image signals with a CCD camera, or capturing digital signals of an image drawn on a mapping board using a piezoelectric elements.
Described above, the conventional organic elements have been generally used as a light emitting display device, and have not been used as a device for capturing image data. Described below, however, the organic EL element has a characteristic for a photosensitive element, so that in combination with the light emitting display function, the organic EL element comes to provide a novel application which has not been achieved.
An object of the present invention is to provide a light emitting display apparatus which has a photosensitive reading function.
The present invention is characterized in that an organic electroluminescence display apparatus comprising: a plurality of organic EL elements arranged on a plane, driving means for driving said plurality of organic EL elements in response to an input signal to emit light from the organic EL elements, light input means for irradiating the organic EL elements with light, and reading means for reading out a voltage generated over the organic EL elements in response to a light input from said irradiating means.
Thus, the organic EL element can have two capabilities for the light emission and photosensitive memory storage, so that the organic EL element serves as a light emitting element and/or a memory, solely or in combination.